See the announcement over on the Technet Forums:Windows XP Service Pack 3 Released to Manufacturing - TechNet Forums
There is a summary of what is in XP SP3 either in PDF or XPS here
‘Windows XP SP3 includes all previously released Windows XP updates, including security updates and hotfixes, and select out-of-band releases. Windows XP SP2 was released in August 2004. Since then, Microsoft has released hundreds of updates. Windows XP SP3 includes all of these updates.
Microsoft is not adding significant Windows Vista functionality to Windows XP through SP3. However, SP3 does include Network Access Protection (NAP) to help organizations that use Windows XP to take advantage of new features in the Windows Server® 2008 operating system’